The present invention relates to a small solid-state laser and an optical transmitter using the solid-state laser.
The number of optical multiplicity in the wavelength-division-multiplexed optical transmission is rapidly increased due to rapid increased demand of an optical transmission capacity. In order to attain the number of wavelength multiplicity, laser diodes of the number of multiplicity are arranged at present (e.g. Hiroshi Toba: O plus E, Vol. 21, No. 8, pp. 965-974). However, if the number of multiplicity is increased in the future, alternative measures are required in consideration of viewpoints of a cost, an apparatus size, the necessity of precise control of a wavelength and the like.
It is an object in the wavelength-division-multiplexed optical transmission to perform reduction of the number of light sources, a cost and an apparatus size and simple control of a wavelength.
In order to achieve the above object, according to the present invention, an ultrashort pulse solid-state laser having a pulse width of 1 picosecond or less is used as a light source of an optical transmitter. Since the ultrashort pulse solid-state laser has a wide spectrum, a large number of wavelength components required for the wavelength-division-multiplexed optical transmission can be taken out from a single laser. Since a wavelength component for each channel is taken out from the ultrashort pulse solid-state laser through a passive element, control of the wavelength is easy. Consequently, a cost is reduced and an apparatus is made small. In order to use the ultrashort pulse solid-state laser as a multi-wavelength light source for the wavelength-division-multiplexed optical transmission, it is necessary to produce a pulse train at a rate matched to an optical transmission rate. In the optical transmission of the mainstay system, transmission is usually made at a rate of 2.4 GHz or 10 GHz, while a general ultrashort pulse solid-state laser produces a pulse train at a repetition rate of about 100 MHz. The ultrashort pulse laser is also improved and the repetition rate of 1.2 GHz is attained recently (Tomaru et al., Opt. Lett. Vol. 25, No. 8, pp. 584-586), while a higher repetition rate is required. For this need, in the present invention, a resonator including only two elements having a gain medium and a solid-state medium or a chirped mirror having a wavelength-dependent dispersion opposite to that of the gain medium is fabricated to attain a small size and the higher repetition rate. When the solid-state laser is operated at the ultrashort-pulse condition, it is necessary to set a dispersion value in the resonator to a proper value and set a minimum beam diameter of a resonator mode in the gain medium to a proper value. The dispersion value can be set by adjusting a length of the solid-state medium or adjusting a chirped value of the chirped mirror. The minimum beam diameter can be set by adjusting a curvature of the curved surface of the gain medium or the solid-state medium.